Python 3.8 multiprocessing behavior breaks pneumatic on macOS

[anthonydb]

Per https://bugs.python.org/issue33725, on macOS the default behavior of the multiprocessing module is spawn instead of fork, which has been deemed unsafe in newer macOS versions. As a result, the way multiprocessing was implemented in pneumatic no longer works; instead, the uploader loops.

Removed multiprocessing via 56654a2 for now.

The benefit of multiprocessing as originally used is questionable since uploads are bound by network capability. At most, it just queues up files faster. Needs research.